org.apache.commons.collections4
Class SetUtils

java.lang.Object
  extended by org.apache.commons.collections4.SetUtils

public class SetUtils
extends Object

Provides utility methods and decorators for Set and SortedSet instances.

Since:
2.1
Version:
$Id: SetUtils.java 1477779 2013-04-30 18:55:24Z tn $

Field Summary
static SortedSet<?> EMPTY_SORTED_SET
          An empty unmodifiable sorted set.
 
Method Summary
static
<T> Set<T>
emptyIfNull(Set<T> set)
          Returns an immutable empty set if the argument is null, or the argument itself otherwise.
static
<E> Set<E>
emptySet()
          Get a typed empty unmodifiable Set.
static
<E> SortedSet<E>
emptySortedSet()
          Get a typed empty unmodifiable sorted set.
static
<T> int
hashCodeForSet(Collection<T> set)
          Generates a hash code using the algorithm specified in Set.hashCode().
static boolean isEqualSet(Collection<?> set1, Collection<?> set2)
          Tests two sets for equality as per the equals() contract in Set.equals(java.lang.Object).
static
<E> Set<E>
orderedSet(Set<E> set)
          Returns a set that maintains the order of elements that are added backed by the given set.
static
<E> Set<E>
predicatedSet(Set<E> set, Predicate<? super E> predicate)
          Returns a predicated (validating) set backed by the given set.
static
<E> SortedSet<E>
predicatedSortedSet(SortedSet<E> set, Predicate<? super E> predicate)
          Returns a predicated (validating) sorted set backed by the given sorted set.
static
<E> Set<E>
synchronizedSet(Set<E> set)
          Returns a synchronized set backed by the given set.
static
<E> SortedSet<E>
synchronizedSortedSet(SortedSet<E> set)
          Returns a synchronized sorted set backed by the given sorted set.
static
<E> Set<E>
transformedSet(Set<E> set, Transformer<? super E,? extends E> transformer)
          Returns a transformed set backed by the given set.
static
<E> SortedSet<E>
transformedSortedSet(SortedSet<E> set, Transformer<? super E,? extends E> transformer)
          Returns a transformed sorted set backed by the given set.
static
<E> Set<E>
unmodifiableSet(Set<E> set)
          Returns an unmodifiable set backed by the given set.
static
<E> SortedSet<E>
unmodifiableSortedSet(SortedSet<E> set)
          Returns an unmodifiable sorted set backed by the given sorted set.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

EMPTY_SORTED_SET

public static final SortedSet<?> EMPTY_SORTED_SET
An empty unmodifiable sorted set. This is not provided in the JDK.

Method Detail

emptySet

public static <E> Set<E> emptySet()
Get a typed empty unmodifiable Set.

Type Parameters:
E - the element type
Returns:
an empty Set

emptySortedSet

public static <E> SortedSet<E> emptySortedSet()
Get a typed empty unmodifiable sorted set.

Type Parameters:
E - the element type
Returns:
an empty sorted Set

emptyIfNull

public static <T> Set<T> emptyIfNull(Set<T> set)
Returns an immutable empty set if the argument is null, or the argument itself otherwise.

Type Parameters:
T - the element type
Parameters:
set - the set, possibly null
Returns:
an empty set if the argument is null

isEqualSet

public static boolean isEqualSet(Collection<?> set1,
                                 Collection<?> set2)
Tests two sets for equality as per the equals() contract in Set.equals(java.lang.Object).

This method is useful for implementing Set when you cannot extend AbstractSet. The method takes Collection instances to enable other collection types to use the Set implementation algorithm.

The relevant text (slightly paraphrased as this is a static method) is:

Two sets are considered equal if they have the same size, and every member of the first set is contained in the second. This ensures that the equals method works properly across different implementations of the Set interface.

This implementation first checks if the two sets are the same object: if so it returns true. Then, it checks if the two sets are identical in size; if not, it returns false. If so, it returns a.containsAll((Collection) b).

Parameters:
set1 - the first set, may be null
set2 - the second set, may be null
Returns:
whether the sets are equal by value comparison
See Also:
Set

hashCodeForSet

public static <T> int hashCodeForSet(Collection<T> set)
Generates a hash code using the algorithm specified in Set.hashCode().

This method is useful for implementing Set when you cannot extend AbstractSet. The method takes Collection instances to enable other collection types to use the Set implementation algorithm.

Type Parameters:
T - the element type
Parameters:
set - the set to calculate the hash code for, may be null
Returns:
the hash code
See Also:
Set.hashCode()

synchronizedSet

public static <E> Set<E> synchronizedSet(Set<E> set)
Returns a synchronized set backed by the given set.

You must manually synchronize on the returned set's iterator to avoid non-deterministic behavior:

 Set s = SetUtils.synchronizedSet(mySet);
 synchronized (s) {
     Iterator i = s.iterator();
     while (i.hasNext()) {
         process (i.next());
     }
 }
 
This method is just a wrapper for Collections.synchronizedSet(Set).

Type Parameters:
E - the element type
Parameters:
set - the set to synchronize, must not be null
Returns:
a synchronized set backed by the given set
Throws:
IllegalArgumentException - if the set is null

unmodifiableSet

public static <E> Set<E> unmodifiableSet(Set<E> set)
Returns an unmodifiable set backed by the given set.

This method uses the implementation in the decorators subpackage.

Type Parameters:
E - the element type
Parameters:
set - the set to make unmodifiable, must not be null
Returns:
an unmodifiable set backed by the given set
Throws:
IllegalArgumentException - if the set is null

predicatedSet

public static <E> Set<E> predicatedSet(Set<E> set,
                                       Predicate<? super E> predicate)
Returns a predicated (validating) set backed by the given set.

Only objects that pass the test in the given predicate can be added to the set. Trying to add an invalid object results in an IllegalArgumentException. It is important not to use the original set after invoking this method, as it is a backdoor for adding invalid objects.

Type Parameters:
E - the element type
Parameters:
set - the set to predicate, must not be null
predicate - the predicate for the set, must not be null
Returns:
a predicated set backed by the given set
Throws:
IllegalArgumentException - if the Set or Predicate is null

transformedSet

public static <E> Set<E> transformedSet(Set<E> set,
                                        Transformer<? super E,? extends E> transformer)
Returns a transformed set backed by the given set.

Each object is passed through the transformer as it is added to the Set. It is important not to use the original set after invoking this method, as it is a backdoor for adding untransformed objects.

Existing entries in the specified set will not be transformed. If you want that behaviour, see TransformedSet.transformedSet(java.util.Set, org.apache.commons.collections4.Transformer).

Type Parameters:
E - the element type
Parameters:
set - the set to transform, must not be null
transformer - the transformer for the set, must not be null
Returns:
a transformed set backed by the given set
Throws:
IllegalArgumentException - if the Set or Transformer is null

orderedSet

public static <E> Set<E> orderedSet(Set<E> set)
Returns a set that maintains the order of elements that are added backed by the given set.

If an element is added twice, the order is determined by the first add. The order is observed through the iterator or toArray.

Type Parameters:
E - the element type
Parameters:
set - the set to order, must not be null
Returns:
an ordered set backed by the given set
Throws:
IllegalArgumentException - if the Set is null

synchronizedSortedSet

public static <E> SortedSet<E> synchronizedSortedSet(SortedSet<E> set)
Returns a synchronized sorted set backed by the given sorted set.

You must manually synchronize on the returned set's iterator to avoid non-deterministic behavior:

 Set s = SetUtils.synchronizedSet(mySet);
 synchronized (s) {
     Iterator i = s.iterator();
     while (i.hasNext()) {
         process (i.next());
     }
 }
 
This method is just a wrapper for Collections.synchronizedSortedSet(SortedSet).

Type Parameters:
E - the element type
Parameters:
set - the sorted set to synchronize, must not be null
Returns:
a synchronized set backed by the given set
Throws:
IllegalArgumentException - if the set is null

unmodifiableSortedSet

public static <E> SortedSet<E> unmodifiableSortedSet(SortedSet<E> set)
Returns an unmodifiable sorted set backed by the given sorted set.

This method uses the implementation in the decorators subpackage.

Type Parameters:
E - the element type
Parameters:
set - the sorted set to make unmodifiable, must not be null
Returns:
an unmodifiable set backed by the given set
Throws:
IllegalArgumentException - if the set is null

predicatedSortedSet

public static <E> SortedSet<E> predicatedSortedSet(SortedSet<E> set,
                                                   Predicate<? super E> predicate)
Returns a predicated (validating) sorted set backed by the given sorted set.

Only objects that pass the test in the given predicate can be added to the set. Trying to add an invalid object results in an IllegalArgumentException. It is important not to use the original set after invoking this method, as it is a backdoor for adding invalid objects.

Type Parameters:
E - the element type
Parameters:
set - the sorted set to predicate, must not be null
predicate - the predicate for the sorted set, must not be null
Returns:
a predicated sorted set backed by the given sorted set
Throws:
IllegalArgumentException - if the Set or Predicate is null

transformedSortedSet

public static <E> SortedSet<E> transformedSortedSet(SortedSet<E> set,
                                                    Transformer<? super E,? extends E> transformer)
Returns a transformed sorted set backed by the given set.

Each object is passed through the transformer as it is added to the Set. It is important not to use the original set after invoking this method, as it is a backdoor for adding untransformed objects.

Existing entries in the specified set will not be transformed. If you want that behaviour, see TransformedSortedSet.transformedSortedSet(java.util.SortedSet, org.apache.commons.collections4.Transformer).

Type Parameters:
E - the element type
Parameters:
set - the set to transform, must not be null
transformer - the transformer for the set, must not be null
Returns:
a transformed set backed by the given set
Throws:
IllegalArgumentException - if the Set or Transformer is null


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